In dental treatment, a prosthesis or an artificial tooth comprising a material made of a metal or the like having a complicated shape that is adaptable to each patient is generally used. As one of the processes for casting a metal that have long been conducted in manufacturing the prosthesis or the like, there is a lost wax process of precision casting. The outline of the process in the case of manufacturing the prosthesis or the like by the process is as follows. In the first place, a part (object) which a patient is in need for is shaped using an impression material, and, based on this, a dental gypsum model is made. And a technician precisely manufactures a model the shape of which is the same as the object from the dental gypsum model with wax by hand. Next, an embedding material comprising a refractory material is poured into the surroundings of the obtained wax pattern (wax model) and hardened, then heating is conducted to form a mold by causing the wax pattern to disappear (to be lost) (incineration or dewaxing). Thereafter a molten metal is poured into the space of the formed mold, and, after cooling, a cast is taken out by breaking the mold to obtain the intended prosthesis or the like having a complicated shape.
Amid the times when the image analysis technology has advanced by leaps and bounds, the following process has been developed and has been utilized in recent years. Specifically, the gypsum model manufactured in the same manner as described above is subjected to three dimensional scanning to obtain a sophisticated image data (hereinafter, also simply referred to as “image data”), the prosthesis or the like being an object is digitally portrayed on a computer (PC), and a resin pattern for the object is output as a mechanically precise stereo image with a 3D printer. It is anticipated that the technology will spread with the advance of the image analysis technology in the future, and in this case, heating is conducted using the pattern manufactured with a resin (an acrylic UV curing resin or the like) in place of the above-described wax pattern to form a mold by causing the resin pattern to disappear (to be lost), and the subsequent production processes are to be conducted.
The embedding materials that are used for the above-described wax or resin pattern are different depending on the kind of metal to be cast, and there exist a “gypsum-based embedding materials”, “phosphate-based embedding materials”, “silica-based embedding materials”, and so on. Among these, the “gypsum-based embedding materials” are used for casting a metal having a relatively lower melting point as compared with the case of using the “phosphate-based embedding materials”. More specifically, in the case where casting is conducted with a metal having a melting point of 1100° C. or less (within a range of noble metal alloy that can be melted with a gas burner) [such as, for example, palladium alloy (such as gold-palladium alloy) and silver], the “gypsum-based embedding materials” are used. Although the “gypsum-based embedding materials” are inferior to the “phosphate-based embedding materials” in casting properties at a high temperature, the “gypsum-based embedding materials” have such advantages that are excellent in a taking-out property of a cast and operability (fluidity) and provides less deformation due to residual stress and less change with time, and are widely utilized.
The “phosphate-based embedding materials” are used in the case where casting is conducted with a metal having a melting point higher than in the case where the “gypsum-based embedding materials” are applied [for example, Co—Cr (cobalt-chromium alloy), Ni—Cr (nickel-chromium alloy, and so on) each having a melting point of 1200° C. to 1400° C.]. Moreover, as in the case where the aforementioned image analysis technology is utilized, conducting casting using a resin pattern in place of a wax pattern has been increased in recent years. Since the resin pattern has a higher disappearance temperature than the wax pattern does, the “phosphate-based embedding material” that is adaptable to a metal having a higher melting point is often used among the embedding materials in the case of casting using a resin pattern.
On the other hand, the heating process in forming a mold by conducting heating to allow the wax pattern to disappear (incineration or dewaxing) (to be lost) in the production process after pouring and hardening the embedding material has been changed in recent years for the purpose of improving the treatment efficiency. Specifically, the heating process has been changed from the “usual heating” in which the temperature of an electric furnace is gradually raised from room temperature to a target temperature to the “rapid heating” in which the embedding material is placed in a furnace having a target temperature to immediately start casting. Therefore, the properties of the embedding material are required so as not to cause cracks, breakage, damage, or the like even when the rapid heating is conducted.
To meet the requirement, since the metals to be used for casting as described above each have a different coefficient of contraction when solidified, any of the embedding materials, when used, is adjusted so as to have a coefficient of expansion to compensate for the coefficient of contraction of the metal to be used. Specifically, the coefficient of expansion of the embedding material is adjusted by, for example, allowing cristobalite or quartz being a heat-expandable refractory material to contain. On the other hand, there is a problem that the embedding material should be the one that is capable of preventing the occurrence of cracks, breakage, or the like liable to occur by the expansion being too large in order for the embedding material to be applicable to the above-described rapid heating. As described previously, since the “gypsum-based embedding materials” have excellent properties but are inferior to the “phosphate-based embedding materials” in casting properties at a high temperature, the “phosphate-based embedding materials” are usually used in the case where casting is conducted through the “rapid heating” using a resin pattern. Here, the incineration temperature in the conventional technology is taken as 700 to 750° C. in the case of using a gypsum-based embedding material and 800 to 900° C. in the case of using a phosphate-based embedding material. However, even though the “phosphate-based embedding material” is used, it is not easy to keep the coefficient of expansion in the optimum state for every metal to be cast and furthermore to suppress the occurrence of cracks or the like that is liable to occur in the case where the “rapid heating” is conducted.
In such present circumstances as described above, when the “phosphate-based embedding material” or the “gypsum-based embedding material” that is adaptable favorably to the “rapid heating” using a resin pattern, particularly the “gypsum-based embedding material” that is excellent in taking out property of a cast is provided, it is extremely useful. Therefore, various proposals the object of which is to provide the above-described “phosphate-based or gypsum-based embedding material” have been made as described below. For example, as a gypsum-based embedding material that does not cause cracks, breakage, damage, or the like to occur even when the rapid heating is conducted, there is a proposal on the gypsum-based embedding material comprising, as main components, calcined gypsum, and cristobalite and quartz each having a particular average particle diameter, to which embedding material an inorganic salt and a powdered refractory material having an average particle diameter larger than the above-described average particle diameter of the cristobalite and the quartz are added as components for increasing air permeability (see, Patent Literature 1). Moreover, there is a proposal on a gypsum-based embedding material for casting comprising a heat-insulating material and hemihydrate gypsum, the gypsum-based embedding material being applicable to casting at a high temperature by adding an MgO—Al2O3 spinel as a heat-insulating material (see, Patent Literature 2). Moreover, there is a proposal that, by adding calcium carbonate to main components comprising hemihydrate gypsum and a heat-insulating material, the air permeability is improved and, as a result thereof, the occurrence of cracks in a mold and burrs in a cast due to generation of a gas through the decomposition of gypsum or wax in calcination at a high temperature are suppressed (see, Patent Literature 3). Moreover, there is also a proposal that, by replacing a part of quartz or cristobalite excellent in performance to compensate for the coefficient of casting contraction of a metal with tridymite in the gypsum-based embedding material or phosphate-based embedding material, the rapid heating of a dental mold is made possible, the time required for the disappearance of a wax pattern and the time required for the preheating of a mold at the time of casting are largely shortened, and casting with high precision is made possible (see, Patent Literature 4). According to the studies made by the present inventors, although the above-described tridymite the rise in the coefficient of thermal expansion of which is calmer when compared with cristobalite, the tridymite does not make any difference from the technology in which cristobalite is used in that the tridymite is a heat-expandable refractory material. Furthermore, in the field of phosphate-based embedding material, there is also a proposal on providing a phosphate-based embedding material that is resistant to heat shock without allowing fissures or cracks to occur event when the rapid heating is conducted (see, Patent Literature 5).